Pulverulent polymers crosslinked on the surface

ABSTRACT

The present invention relates to a pulverulent polymer post-crosslinked on the surface and absorbing water or aqueous liquids, synthesised from polymerised, optionally pre-crosslinked monomers containing partially neutralised carboxyl groups. The present invention also relates to a process for the post-treatment of the aforementioned polymers and the use of a solution of at least one salt of an at least trivalent cation for restoring the gel permeability of the aforementioned polymers that have been damaged by mechanical action.

FIELD OF THE INVENTION

The present invention relates to a pulverulent polymer post-crosslinkedon the surface that absorbs water or aqueous liquids, synthesised frompolymerised, optionally pre-crosslinked, partially neutralised monomerscontaining carboxyl groups. The present invention also relates to aprocess for the post-treatment of the aforementioned polymers and theuse of a solution of at least one trivalent cation for the restorationof the gel permeability of the aforementioned polymers that have beendamaged by mechanical action.

BACKGROUND OF THE INVENTION

Polymers that absorb aqueous liquids, so-called superabsorbers, areknown from numerous publications. Modified natural polymers as well aspartially or completely synthetic polymers may be used for this purpose.The fully synthetic polymers are, as a rule, produced by free-radicalpolymerisation of various hydrophilic monomers in aqueous solutionaccording to different methods. In general, crosslinking agents areincorporated by polymerisation, whereby the polymer obtained is nolonger water-soluble but is only water-swellable. For example, polymersbased on (meth)acrylic acid that are present in partially neutralisedform as alkali metal salt may be used as superabsorbers.

The superabsorber polymer is, as a rule, mechanically comminuted(reduced to a powder), dried and ground after the polymerisation. Inthis connection the pulverulent, water-swellable polymer falls in a moreor less broad grain spectrum depending on the production process, whichis typically in a range from 10 to 1000 μm, of which normally the grainfraction from 150 to 850 μm is used for practical purposes, above all inthe hygiene sector, as absorption material. Fine fractions of less than150 μm are undesirable on account of their dust-forming behaviour andtheir toxic properties when inhaled.

The development of more recent nappy constructions is highlighted by thetendency to replace increasingly larger proportions of the voluminouscellulose fluff by superabsorbers. This is happening on grounds ofvolume reduction and of an improved property profile. Due to theincreased concentration of superabsorbers there is increased contact ofthe swollen absorber particles with one another after absorption ofliquid has taken place. By means of the surface post-crosslinkingmethods described in the prior art, the so-called gel blocking, in whichonly the surfaces of the absorber particles swell and the liquid doesnot penetrate into the inner regions and swollen absorber particles thathave clumped together build up to form a barrier layer for subsequentliquid, can be suppressed.

The superabsorbers are therefore post-crosslinked on the surface afterthe comminution, drying, grinding and grading.

Such surface post-crosslinking processes are described for example inpatent specifications DE 40 20 780 C1 and U.S. Pat. No. 4,043,952. In DE40 20 780 C1 the polymers are crosslinked on the particle surface by lowmolecular weight organic compounds. Not only is the absorptive capacityunder pressure thereby raised, but also the behaviour of the absorbersknown as “gel blocking” is suppressed. U.S. Pat. No. 4,043,952 disclosesa treatment of the surface of the absorber particles with at leastdivalent metal ions (column 8, line 51) in organic solvents in order toimprove the dispersibility in aqueous media and effect a more rapidabsorption of the liquid. EP 233 067 B1 and U.S. Pat. No. 4,558,091describe the surface post-crosslinking of superabsorbers based onpolyacrylic acid and/or hydrolysed starch/acrylonitrile graft polymerswith aluminium compounds in combination with polyhydroxyalcohols inorder to improve the absorption properties.

During and after the surface post-crosslinking the polymer powders arechanged by mixing and transportation processes as regards their grainspectrum, due to the formation of finely particulate abraded material,with the result that a renewed screening of the fine fractions isnecessary in order to restore the previous state. This results inadditional production costs and material losses since the fine fractionscan now no longer be used at all, or at best only to a limited extent.In addition to the formation of the abraded material, there is also adeterioration of the absorption properties that had previously beenimproved by the surface post-crosslinking, i.e. in particular theability of the swollen absorber gel to transport further liquid (gelpermeability) is also impaired. This problem occurs not only in theproduction of the superabsorber powders, but ultimately also in theirsubsequent further processing for the production of hygiene products. Inthis case it is frequently found that the absorption properties of thesuperabsorbers are impaired due to abrasion during their conveyance, andundesirable dust is formed.

In EP 691 995 A1 describes measures for example for reducing theproportion of dust by addition of polyglycols, which however onlyprevent the dust but do not deal with the problem of the deterioratedabsorption properties. U.S. Pat. No. 5,002,986 describes a process forimproving the absorption rate of superabsorbers that consist only offine fractions that are agglomerated in intensive mixers in the presenceof ionic crosslinking agents to form larger particles. DE 196 46 484 A1describes superabsorbers consisting of a combination of specialcrosslinking agents and monomers that suffer only a slight deteriorationin properties under mechanical stress. A significant suppression of theproperty loss cannot however be achieved in this way.

In the prior art, no superabsorbing polymer is known whose propertiesare not impaired by mechanical stress during conveyance in productionand nappy manufacture. The prior art also does not disclose any processthat solves the problem of impairment of the properties of thesuperabsorber powders due to the mechanical stress during the surfacemodification and subsequent conveyance in production and nappymanufacture. In any case, a screening of fine fractions is carried outat the expense of product yield.

The object of the present invention is to provide polymers whoseproperties deteriorate only insignificantly during nappy manufacture,that do not form dust or only small amounts of dust, and that have alesser tendency to form clumps in environments with a high atmosphericmoisture content than products of the prior art. The object of thepresent invention is also to provide a process for the production of theaforementioned polymers, by means of which the screening of the finefractions after the surface post-crosslinking is largely avoided withoutthe properties of the polymer being substantially impaired. A furtherobject of the present invention is to provide a substance by means ofwhich the gel permeability of surface-crosslinked superabsorber powderscontaining fine fractions and that have been damaged by abrasionprocesses is restored.

SUMMARY OF THE INVENTION

This object is achieved according to the invention by a pulverulentpolymer post-crosslinked on the surface that absorbs water or aqueousliquids, and that is synthesised from polymerised, optionallypre-crosslinked monomers containing partially neutralised carboxylgroups, wherein the pulverulent polymer has, after thepost-crosslinking, been reacted with the preferably aqueous solution ofat least one salt of an at least trivalent cation.

The polymers according to the invention have, compared to superabsorbersaccording to the prior art, improved gel permeabilities that correspondroughly to those of mechanically unstressed polymers. The retentionbehaviour is not reduced in the polymer according to the invention andthe swellability under pressure is not impaired or only slightly so.Furthermore, it has been found that the polymers according to theinvention have a good flowability and exhibit an improvement in theso-called anticaking behaviour, i.e. in a moist ambient atmosphere theyhave only a slight tendency to agglomerate and moreover exhibit areduced dust formation. Accordingly, the treatment steps that arenormally envisaged for the dust removal and anticaking treatment can bereduced or dispensed with entirely. The pulverulent polymers accordingto the invention permit a reliable further processing for example intonappies without dust formation and loss of properties during mechanicalor pneumatic conveyance.

DETAILED DESCRIPTION OF THE INVENTION

The salt component that may be used in the solution according to theinvention include chlorides, bromides, sulfates, carbonates, nitrates,phosphates as well as salts of organic acids, such as for exampleacetates and lactates, and other salts of at least trivalent cations.Examples of cations that may be used according to the invention includealuminium as well as iron, chromium, manganese, titanium, zirconium andother transition metals as well as double salts of two cations or alsomixtures of several salts. Aluminium salts and alums and their varioushydrates are preferred, such as for example AlCl₃×6H₂O, NaAl(SO₄)₂×12H₂O, KAl(SO₄)₂×12 H₂O or Al₂(SO₄)₃×18 H₂O. Al₂(SO₄)₃ and its hydratesare particularly preferably used. The salt components are preferablyused, calculated on the basis of the cation according to the invention,in amounts of about 0.001 to about 1.0 wt. %, preferably about 0.002 toabout 0.5 wt. % and particularly preferably about 0.005 to about 0.2 wt.%, in each case referred to the polymer. The added amount is preferablycalculated so that the absorption of the polymer powder under pressureis not, or only slightly, impaired.

The salts of at least trivalent cations to be used according to thepresent invention are preferably applied in the form of a solution.Suitable solvents are water or polar, water-miscible organic solventssuch as for example acetone, methanol, ethanol or 2-propanol or theirmixtures; water is preferably used. The term aqueous solution within thecontext of the invention means in relation to the solvent component thatthe solution may contain, apart from water, also other organic solvents.The concentration of the salts (calculated in anhydrous form) in thesolvent may vary within wide limits and is generally in the range fromabout 1 to about 80 wt. %, preferably in a range from 1 to 60 wt. % andmost particularly preferably in a range from about 5 to about 35 wt. %.The preferred solvent for the salt component is water, which is used inan amount of about 0.05 to about 10 wt. %, preferably about 0.1 to about5 wt. % and particularly preferably about 0.1 to about 3 wt. %, referredto the polymer. The amount of water is preferably adjusted in the lowerrange so that sufficient liquid is available to distribute the saltsolution. In the upper range the amount of water must on the other handbe optimised so that the formation of agglomerates, which may occurtemporarily when using relatively large amounts of water, remains withinacceptable limits. It is generally the case that, with increasing amountof salt of at least trivalent cations, increasing amounts of water mayalso be used without resulting in a temporary agglomerate formation.

Natural, partially synthetic and fully synthetic substances are suitableas water-swellable hydrophilic polymers. Partially synthetic and fullysynthetic substances are preferred, in particular anionic polymers basedon (meth)acrylic acid that are present in partially neutralised form asalkali metal salts, in particular sodium and/or potassium salts. Thedegree of neutralisation of the acid monomer components may vary, but ispreferably between about 25 and about 85 mol %. These components may behomopolymers and copolymers that can be obtained from acrylic acidand/or methacrylic acid alone, from these monomers together with one ormore other monomers, or simply from one or more other monomers. Examplesinclude grafted-on anionic polymers based on (meth)acrylic acid, presentin partially neutralised form as alkali metal salt and graft polymers onpolyvinyl alcohols, on polysaccharides such as for example starch orcellulose or derivatives thereof, or on polyalkylene oxides such aspolyethylene oxides or polypropylene oxides.

Monomers that may be used in addition to (meth)acrylic acid in theproduction of the polymers include methyl, ethyl and (poly)hydroxyalkylesters of (meth)acrylic acid, (meth)acrylamide, crotonic acid, maleicand fumaric acids, itaconic acid, 2-acrylamido-2-methylpropanesulfonicacid, vinylsulfonic acid and vinylphosphonic acid and the methyl, ethyland (poly)hydroxyalkyl esters and amides of these acids, aminogroup-containing and ammonium group-containing esters and amides of allthe aforementioned acids and water-soluble N-vinylamides. Also thepolymer may contain structural units derived from all monomersconventionally used in the production of superabsorber polymers. Thepolymer is preferably crosslinked.

Suitable crosslinking substances may contain two or more reactive groupsthat may be used in the production of the superabsorber polymers, whosestructural units may then be contained in the polymer, which includepolyglycidyl ethers, methylene bis(meth)acrylamide, bisacrylamidoaceticacid, esters of unsaturated acids of polyols and/or alkoxylated polyols,for example ethylene glycol di(meth)acrylate or trimethylolpropanetriacrylate or allyl compounds, such as for example allyl(meth)acrylate, polyallyl esters, tetraallyloxyethane, triallylamine,tetraallyl ethylenediamine or allyl esters of phosphoric acid as well asvinylphosphonic acid derivatives. The proportion of crosslinking agentsthat are already added in the production of the superabsorber polymersis preferably about 0.01 to about 20 wt. %, particularly preferablyabout 0.1 to about 3 wt. %, referred to the total monomers used.

The production of the polymer is otherwise carried out according to wellknown methods, such as are described for example in DE 40 20 780 C1,which is hereby incorporated by reference. Preferably the production isdone by polymerisation in aqueous solution according to the so-calledgel polymerisation process.

The polymer powders are formed by comminution, drying and grinding ofthe polymer gels followed by surface post-crosslinking and may have abroad grain spectrum. Suitable substances for such a surfacepost-crosslinking are compounds containing two or more groups that canform covalent bonds with the carboxyl groups of the hydrophilic polymer.Suitable compounds are diols and polyols, diglycidyl compounds andpolyglycidyl compounds such as phosphonic acid diglycidyl esters,alkylene carbonates such as ethylene carbonate, alkoxysilyl compounds,polyaziridines, polyamines or polyamidoamines, in which connection theaforementioned compounds may also be used in the form of mixtures withone another.

The surface post-crosslinking agent is used in an amount of about 0.01to about 30 wt. %, preferably about 0.1 to about 10 wt. %, referred tothe polymer to be post-crosslinked.

Before the surface post-crosslinking the polymer is preferably dried,ground and screened to the grain fraction appropriate for the respectiveapplication technology, and is then added to the surfacepost-crosslinking reaction. In many cases it has however also provedconvenient to add the surface post-crosslinking agents already beforethe drying of the polymer gel and/or before the comminution of thepartially or largely dried polymer. A surface post-crosslinking to becarried out according to the invention is described for example in U.S.Pat. No. 4,666,983 and in DE 40 20 780. These specifications are herebyintroduced by way of reference. The addition of the surfacepost-crosslinking agents takes place in the form of a solution in water,organic solvents or mixtures thereof, especially if minor amounts ofsurface post-crosslinking agents are used. Suitable mixing units forapplying the surface post-crosslinking agent are for examplePatterson-Kelley mixers, DRAIS turbulence mixers, Lodige mixers, Rubergmixers, screw mixers, pan mixers and fluidised bed mixers, as well ascontinuously operating vertical mixers in which the powder is mixed athigh speed by means of rotating blades (Schugi mixers). After thesurface post-crosslinking agent has been mixed with the pre-crosslinkedpolymer, the reaction mixture is heated to temperatures of about 60° toabout 250° C., preferably to about 135° to about 200° C. andparticularly preferably to about 150° to about 185° C. in order to carryout the surface post-crosslinking reaction. The duration of thepost-heating is limited by the point at which the desired propertyprofile of the polymer is destroyed again as a result of heat damage.

After the surface post-crosslinking step has been completed, thepulverulent polymer is reacted with the solution of at least one salt ofan at least trivalent cation. The moisture content of the polymer powderbefore the reaction may fluctuate, and is typically less than about 10wt. %, preferably less than about 8 wt. % and particularly preferablyless than about 5 wt. %.

The polymer powder may contain fine fractions before the reaction. Thisfine fraction may be formed during the drying, grinding and/orpost-crosslinking or may be added to the polymer powder, so that thepolymer according to the invention may also contain recycled finefractions. Preferably, the fine dust fraction with a mean grain diameterof less than about 150 μm is up to about 15 wt. %, particularlypreferably up to about 10 wt. % and most particularly preferably up toabout 5 wt. %. In order for the polymer powders to be used in thehygiene industry, an upper grain limit of about 1000 μm, preferably ofabout 850 μm, has proved suitable.

The powder and the solution of at least one salt of the at leasttrivalent cation are preferably intimately and as homogeneously aspossible mixed with one another and thereby reacted.

The pulverulent water-swellable hydrophilic polymers according to theinvention may be used for all purposes for which such superabsorbers arenormally employed, in particular for the absorption of water and aqueoussolutions. They are preferably used in the absorption of body fluids, inparticular blood and urine. For this they are incorporated in particularin absorbing single-use disposable hygiene articles, for example innappies, tampons or sanitary towels, or also for other medicinalpurposes. Other possible uses include for example as water-storing floorimprovement agents or as moisture binding agents for cable sheathing oras support material for active constituents and their controlledrelease.

The present invention also provides a process for the production of thepolymers according to the invention, in which a solution of at least onesalt of at least a trivalent cation is added to the pulverulent polymerafter the post-crosslinking and the said pulverulent polymer and thesolution are preferably homogeneously thoroughly mixed.

By means of the process according to the invention it is possible torestore the gel permeability of polymer powders that have suffered highabrasion damage. The stage involving the screening of the fine fractionsafter the post-crosslinking is omitted. The process products have,compared to superabsorbers according to the prior art, improved gelpermeabilities that correspond approximately to those of mechanicallyunstressed polymers. The retention behaviour is not adversely affectedby the process according to the invention and the swellability underpressure is not reduced or only slightly so. Furthermore it has beenfound that the pulverulent process products have a good flowability, animprovement in the so-called anticaking behaviour, i.e. only a slighttendency to agglomerate in a moist ambient atmosphere, and furthermorehave a reduced tendency to form dust. The treatment steps that arenormally envisaged for dust removal and anticaking behaviour may bereduced or dispensed with altogether. The polymer powders according tothe invention permit a reliable further processing without formation ofdust and loss of properties during mechanical or pneumatic conveyance. Afurther advantage of the process according to the invention is the factthat the solvent fraction introduced via the salt solution does not haveto be distilled off, especially if it consists only of water, with theresult that the polymer powder can be used without further working-up.

According to the invention a solution of at least one salt of an atleast trivalent cation is added to the polymer powder after thepost-crosslinking. The pulverulent polymer and the solution arepreferably homogeneously thoroughly mixed during or after the additionof the solution.

The moisture content of the polymer powder after the post-crosslinkingand before the addition of the solution may vary, and is typically lessthan about 10 wt. %, preferably less than about 8 wt. % and particularlypreferably less than about 5 wt. %.

The pulverulent polymer may contain very fine dust fractions before theaddition of the solution, which are formed either in thepost-crosslinking or are added to the polymer powder, with the resultthat the process according to the invention is also suitable for therecycling of fine fractions. Preferably the grain fraction of less thanabout 150 μm accounts for up to about 15 wt. %, preferably up to about10 wt. % and most preferably up to about 5 wt. %. An upper grainboundary of about 1000 μm, preferably about 850 μm, has proved suitablefor the use of the polymer powders in the hygiene industry.

According to the invention the powder must be mixed intimately and ashomogeneously as possible with the solution of the salt of the at leasttrivalent cation. The mixing may be carried out in a continuous ordiscontinuous procedure in any apparatus suitable for mixing pulverulentproducts with liquid additives. The mixing is preferably carried outusing a stirrer mixer that is preferably operated at a rotational speedof about 700 to about 1000 r.p.m. In particular renewed damage to theprocess products is thereby avoided. The intensive mixers with a highenergy input of for example about 1000 to about 5000 Wh/m³ that areotherwise described in the prior art (DE 41 31 045 C1) for the surfacetreatment of superabsorbing polymer powders should preferably not beused for the process according to the invention.

The mixing times are generally between 1 and about 120 minutes, butpreferably less than about 1 hour. Temporary agglomerates, such as maybe formed by the addition of relatively large amounts of water, arebroken down again by the gentle mixing movement of the mixer, but canhowever prolong the mixing time.

The addition of the solution to the pulverulent polymers takes placepreferably in the temperature range from 0° C. to about 100° C.,particularly preferably in the range from about 10° C. to about 80° C.,most particularly preferably in the range from about 20° C. to about 50°C.

The present invention furthermore provides the polymers that are formedby the process according to the invention.

The pulverulent water-swellable hydrophilic polymers according to theinvention may be used for all purposes for which such superabsorbers arenormally employed, in particular therefore for the absorption of waterand aqueous solutions. They are preferably used in the absorption ofbody fluids, in particular blood and urine. For this they areincorporated in particular in absorbing single-use disposable hygienearticles, for example in nappies, tampons or sanitary towels, or alsofor other medicinal purposes. Other possible uses include for example aswater-storing floor improvement agents or as moisture binding agents forcable sheathing or as support material for active constituents and theircontrolled release.

The present invention furthermore provides for the use of a solution ofat least one salt of an at least trivalent cation for the restoration ofthe gel permeability of pulverulent polymers absorbing water or aqueousliquids that have been damaged by mechanical action, synthesised frompolymerised, optionally pre-crosslinked, partially neutralised monomerscontaining carboxyl groups, in which the solution of the salt is addedto the pulverulent polymer after the post-crosslinking and thepulverulent polymer and the solution are thoroughly mixed.

By means of the use according to the invention it is possible to restorethe gel permeability of polymer powders that have suffered high abrasiondamage and with large fine fractions. The stage involving the screeningof the fine fractions after the post-crosslinking is omitted. Theprocess products have, compared to superabsorbers according to the priorart, improved gel permeabilities that correspond approximately to thoseof mechanically unstressed polymers. The retention behaviour is notadversely affected by the process according to the invention and theswellability under pressure is not reduced or only slightly so.Furthermore it has been found that the pulverulent process products havea good flowability, an improvement in the so-called anticakingbehaviour, i.e. only a slight tendency to agglomerate in a moist ambientatmosphere, and furthermore have a reduced tendency to form dust.

The treatment steps that are normally envisaged for dust removal andanticaking behaviour may therefore be reduced or dispensed withaltogether. The polymer powders according to the invention permit areliable further processing, for example into nappies, without formationof dust and loss of properties during mechanical or pneumaticconveyance. A further advantage of the process according to theinvention is the fact that the solvent fraction introduced via the saltsolution does not have to be distilled off, especially if it consistsonly of water, with the result that the polymer powder can be usedwithout further working-up.

The addition of the solution to the pulverulent polymer takes place inthe temperature range from 0° C. to about 100° C., particularlypreferably in the range from about 10° C. to about 80° C., mostparticularly preferably in the range from about 20° C. to about 50° C.Temperatures above 100° C. and/or subsequent temperature treatments areundesirable since they either do not produce any improvement of theproperties or may even lead to a deterioration of the properties.

The polymers according to the invention as well as the superabsorbersoccurring as a result of the process according to the invention or theuse according to the invention are preferably employed inliquid-absorbing hygiene products such as for example baby nappies,incontinence products and sanitary towels.

Liquid-absorbing hygiene products as a rule are generally constructed ofa liquid-permeable covering facing the body, a liquid-absorbing suctionlayer as well as a substantially liquid-impermeable outer layer facingaway from the body. Optionally further structures are also used for therapid absorption and distribution of body fluid in the suction core.These structures are frequently but however not necessarily used betweenthe liquid-permeable covering facing the body and the liquid-absorbingsuction layer.

The liquid-permeable covering consists as a rule of a non-wovenfibre-like fleece or another porous structure. Suitable materials forthis covering include for example synthetic polymers such as polyvinylchloride or fluoride, polytetrafluoroethylene (PTFE), polyvinyl alcoholsand derivatives, polyacrylates, polyamides, polyesters, polyurethanes,polystyrene, polysiloxanes or polyolefins (e.g. polyethylene (PE) orpolypropylene (PP)) as well as natural fibre materials and alsoarbitrary combinations of the aforementioned materials in the sense ofmixed materials or composite materials or copolymers.

The liquid-permeable covering has a hydrophilic character. It mayfurthermore consist of a combination of hydrophilic and hydrophobicconstituents. As a rule the liquid-permeable covering is preferablyrendered hydrophilic in order to ensure that body fluids are rapidlysoaked up into the liquid-absorbing suction layer, although partiallyhydrophobised coverings are also employed.

Liquid-absorbing Suction Layer

The liquid-absorbing suction layer contains the superabsorbing powdersand/or granules and further components of for example fibrous materials,foam-like materials, film-forming materials or porous materials, as wellas combinations or two or more of these materials. Each of thesematerials may either be of natural or synthetic origin or may have beenproduced by chemical or physical modification of natural materials. Thematerials may be hydrophilic or hydrophobic, hydrophilic materials beingpreferred. This applies in particular to those compositions that areintended efficiently to absorb excreted body fluids and transport thelatter in the direction of regions of the absorbing core more remotefrom the entry point of the body fluid.

Suitable hydrophilic fibre materials include for example cellulosefibres, modified cellulose fibres (e.g. stiffened cellulose fibres),polyester fibres (e.g. Dacron), hydrophilic nylon but also hydrophilisedhydrophobic fibres such as for example polyolefins (PE, PP), polyesters,polyacrylates, polyamides, polystyrene, polyurethanes, etc., that havebeen hydrophilised with surfactants.

Cellulose fibres and modified cellulose fibres are preferably used.Combinations of cellulose fibres and/or modified cellulose fibres withsynthetic fibres such as for example PE/PP composite materials,so-called bicomponent fibres, such as are used for example for thethermobonding of airlaid materials or other materials are also commonlyused. The fibre materials may be present in various application forms,for example as loose cellulose fibres precipitated from an airstream orfrom an aqueous phase, or laid cellulose fibres, as non-woven fleece oras tissue. Combinations of various application forms are possible.

In addition to the superabsorbers according to the invention there mayoptionally be used further pulverulent substances, such as for exampledeodorising substances such as cyclodextrines, zeolites, inorganic ororganic salts, and similar materials.

As porous materials and foam-like materials there may for example beused polymer foams such as are described in the specifications DE 44 18319 A1 and DE 195 05 709 A1, which are hereby incorporated by reference.

Thermoplastic fibres (e.g. two-component fibres formed from polyolefins,polyolefin granules, latex dispersions or hot-melt adhesives) may beused for the mechanical stabilisation of the liquid-absorbing suctionlayer. Optionally one or more tissue plies are used for thestabilisation.

The liquid-absorbing suction layer may be a single-ply layer or mayconsist of several layers. Preferably structures are used that consistof hydrophilic fibres, preferably cellulose fibres, optionally having astructure for the rapid absorption and distribution of body fluids, suchas for example chemically stiffened (modified) cellulose fibres orhighloft fleeces of hydrophilic or hydrophilised fibres as well assuperabsorbing polymers.

The superabsorbing polymer according to the invention may be distributedhomogeneously in the cellulose fibres or the stiffened cellulose fibres,may be incorporated in the form of plies between the cellulose fibres orthe stiffened cellulose fibres, or the concentration of thesuperabsorbing polymer may exhibit a gradient within the cellulosefibres or stiffened cellulose fibres. The ratio of the total amount ofsuperabsorbing polymer to the total amount of cellulose fibres orstiffened cellulose fibres in the absorbing suction core may varybetween 0 to about 100% and about 80 to about 20%, wherein in oneembodiment concentrations of up to 100% of superabsorber may be achievedlocally, for example with a gradient-type or layer-type incorporation.Such structures with regions of high concentrations of absorbingpolymer, in which the proportion of superabsorber in specific regions isbetween about 60% and about 100% and most preferably between about 90%and about 100%, are also described for example in U.S. Pat. No.5,669,894, which is hereby incorporated by reference.

Optionally several different superabsorbers differing for example insuction rate, permeability, storage capacity, absorption under pressure,grain distribution or also chemical composition, may also be usedsimultaneously. The various superabsorbers may be introduced, mixed withone another, into the suction cushion or alternatively may bedistribution in a locally differentiated manner in the absorbent core.Such a differentiated distribution may take place over the thickness ofthe suction cushion or over the length or width of the suction cushion.

The liquid-absorbing suction layer accommodates one or more of theaforedescribed plies of cellulose fibres or stiffened cellulose fibrescontaining superabsorbing polymers. In a preferred embodiment structuresare used consisting of combinations of plies with a homogeneousintroduction of superabsorber and in addition a layer-typeincorporation.

Optionally these aforementioned structures are also complemented byfurther plies of pure cellulose fibres or stiffened cellulose fibres onthe side facing the body and/or also on the side facing away from thebody.

The aforedescribed structures may also be multiply repeated, which mayinvolve a layer formation of two or more identical layers on top of oneanother or alternatively a layer formation of two or more differentstructures on top of one another. In this connection the differences arein turn of a purely structural nature or may also relate to the type ofmaterial used, such as for example the use of absorbing polymers ordifferent types of cellulose differing as regards their properties.Optionally the whole suction cushion or also individual plies of theliquid-absorbing suction layer are separated by plies of tissue of othercomponents, or are in direct contact with other plies or components.

For example, the structure for the rapid absorption and distribution ofbody fluids and the liquid-absorbing suction layer may be separated fromone another by tissue or may however be in direct contact with oneanother. Provided there is no separate structure for the rapidabsorption and distribution of body fluids between the liquid-absorbingsuction layer and the liquid-permeable covering facing the body, butinstead the liquid distribution effect is to be achieved for example bythe use of a special liquid-permeable covering facing the body, then thesaid liquid-absorbing suction layer may likewise optionally be separatedfrom the liquid-permeable covering facing the body by a tissue.

Instead of tissue, non-woven fleece may optionally also be incorporatedinto the liquid-absorbing suction layer. Both components lead to thedesired secondary effect of stabilising and securing the absorption corein the moist state.

Process for Producing the Liquid-absorbing Suction Layer

Fibre-containing and superabsorber-containing layers that distribute andstore liquid can be made by a large number of production processes.

Apart from the established conventional processes, such as are generallyknown to the person skilled in the art, involving drum forming with theaid of moulded wheels, pockets and product moulds and correspondinglyadapted metering devices for the raw materials, there may also be usedmodern established processes such as the airlaid process (e.g. EP 850615, column 4 line 39 to column 5 line 29, U.S. Pat. No. 4,640,810) withall forms of metering, laying of the fibres and compaction such ashydrogen bonding (e.g. DE 197 50 890, column 1 line 45 to column 3 line50, thermobonding, latex bonding (e.g. EP 850 615, column 8 line 33 tocolumn 9 line 17 and hybrid bonding, the wetlaid process (e.g. PCT WO99/49905, column 4 line 14 to column 7 line 16), carding, meltblown,spunblown processes as well as similar processes for the production ofsuperabsorber-containing non-wovens (within the meaning of the EDANAdefinition, Brussels), also in combinations of these processes withconventional methods for the production of the aforementioned liquidstorage media. The aforementioned specifications are incorporated byreference.

Further suitable processes include the production of laminates in thebroadest sense, as well as the production of extruded and co-extruded,wet-compacted and dry-compacted and also subsequently compactedstructures.

Combinations of these processes with one another is also possible.

Structures for the Rapid Absorption and Distribution of Body Fluid

A structure for the rapid absorption and distribution of body fluidconsists for example of chemically stiffened (modified) cellulose fibresor highloft fleeces of hydrophilic or hydrophilised fibres or acombination of the two.

Chemically stiffened, modified cellulose fibres may be produced forexample from cellulose fibres that have been converted in a chemicalreaction by means of crosslinking agents such as for example C₂-C₈dialdehydes, C₂-C₈ monoaldehydes with an additional acid function, orC₂-C₈ polycarboxylic acids. Particular examples include glutaraldehyde,glyoxal, glyoxalic acid or citric acid. Also known are cationicallymodified starch or polyamide-epichlorohydrin resins (e.g. KYMENE 557Hfrom Hercules Inc., Wilmington, Del.). A twisted crumpled structure isproduced and stabilised by the crosslinking, which acts advantageouslyon the rate of liquid absorption.

Weight Per Unit Area and Density of Liquid-absorbing Articles

The absorbing hygiene products may differ as regards their weight perunit area and thickness and accordingly the density may vary greatly.Typically the densities of the regions of the absorption cores arebetween about 0.08 and about 0.25 g/cm³. The weights per unit areas arebetween about 10 and about 1000 g/m², preferred weights per unit areabeing between about 100 and about 600 g/m² (see also U.S. Pat. No.5,669,894, which is incorporated by reference). The density varies as arule over the length of the absorbing core, as a result of a selectivemetering of the amount of cellulose fibres or stiffened cellulose fibresor of the amount of the superabsorbing polymer, since these componentsin preferred embodiments are incorporated more strongly into the frontregion of the absorbing disposable article.

This selective increase in the absorbing material in specific regions ofthe absorbing core may also be achieved in another way by for exampleproducing an appropriately large two-dimensional structure by means ofan airlaid or wetlaid process, the said structure consisting ofhydrophilic cellulose fibres, optionally of stiffened cellulose fibres,optionally of synthetic fibres (e.g. polyolefins) as well as ofsuperabsorbing polymers, which are then folded or laid on top of oneanother.

Test Methods

Test Method 1: Retention (TB)

The retention is measured by the teabag method and is given as the meanvalue of three measurements. About 200 mg of polymer are welded into ateabag and immersed for 30 minutes in 0.9% NaCl solution. The teabag isthen centrifuged for 3 minutes in a centrifuge (23 cm diameter, 1,400r.p.m.) and weighed. A teabag without water-absorbing polymer is alsocentrifuged to provide a blank value.Retention=final weight−blank value/initial weight [g/g]Test Method 2: Anticaking Test and Moisture Absorption

This test is intended to serve for the evaluation of the cakingbehaviour and to determine the moisture absorption. For this, 5 g ofsuperabsorber are weighed out into a dish and uniformly distributed, andstored for more than 3 hours at about 38° C. and about 80% relativeatmospheric humidity in a climatic test cabinet. The dish is thenreweighed in order to determine the moisture uptake. The cakingbehaviour is determined from the percentage screenings of thesuperabsorber sample through a screen of 1.68 mm mesh width shaken threetimes. Superabsorbers with a good anticaking behaviour exhibit only aslight tendency to agglomerate when stored under moist conditions andpass almost completely through the screen.

Test Method 3: Gel Permeability (SFC)

The test is carried out according to a method published in WO 95/22356,which is incorporated herein by reference. About 0.9 g of superabsorbermaterial is weighed out into a cylinder with a screen plate andcarefully distributed over the screen surface. The superabsorbermaterial is allowed to swell in JAYCO synthetic urine for 1 hour againsta pressure of about 20 g/cm². After measuring the swelling height of thesuperabsorber 0.118 M NaCl solution is allowed to flow at constanthydrostatic pressure from a levelled storage vessel through the swollengel layer. The swollen gel layer is covered with a special screencylinder during the measurement, which ensures a uniform distribution ofthe 0.118 M NaCl solution above the gel and constant conditions(measurement temperature 20°-25° C.) as regards the state of the gel bedduring the measurement. The pressure acting on the swollen superabsorberis 20 g/cm². With the aid of a computer and a weighing machine theamount of liquid passing through the gel layer as a function of time ismeasured at 20-second intervals over a period of 10 minutes. The flowrate (in g/sec) through the swollen gel layer is determined by means ofregression analysis by extrapolation of the gradient and determinationof the midpoint at time t=0 of the amount of flow within the period 2-10minutes. The SFC value (K) is calculated as follows:$K = {\frac{{F_{s}\left( {t = 0} \right)} \cdot L_{0}}{{r \cdot A \cdot \Delta}\quad P} = \frac{{F_{s}\left( {t = 0} \right)} \cdot L_{0}}{139506}}$where: F_(s)(t=0) is the flow rate in g/sec

-   -   L₀ is the thickness of the gel layer in cm    -   r is the density of the NaCl solution (1.003 g/cm³) A is the        area of the surface of the gel layer in the measurement cylinder        (28.27 cm²)    -   ΔP is the hydrostatic pressure acting on the gel layer (4920        dynes/cm²), and K is the SFC value [cm³·s·g⁻¹]        Test Method 4: Liquid Absorption Under Pressure (AAP Test)

The absorption under pressure (pressure load 50 g/cm²) is determinedaccording to a method described in EP 0339461, page 7. About 0.9 g ofsuperabsorber is weighed out into a cylinder with a screen plate. Theuniformly scattered layer of superabsorber is tamped with a pestle,which exerts a pressure of 50 g/cm². The previously weighed cylinder isthen placed on a glass filter plate arranged in a dish containing 0.9%NaCl solution whose liquid level corresponds exactly to the height ofthe filter plate. After the cylinder unit has absorbed the 0.9% NaClsolution for one hour, the cylinder unit is reweighed and the AAP iscalculated as follows:

AAP=final weight (cylinder unit+fully soaked superabsorber)−initialweight (cylinder unit+superabsorber)/weighed out amount ofsuperabsorber.

Test Method 5: Flowability (FFC Value)

The flowability of the superabsorbing polymer powders is determined withthe RST-1.01 annular shear device from Dr.-Ing Dietmar SchulzeSchüttgutmeβtechnik. The FFC value provides information on the flowproperties of a bulk material in a silo. In the measurement the bulkmaterial is subjected to various loads in an annular shear cell (initialshear load 500,000 Pa, shearing off loads 100,000 Pa, 250,000 Pa,400,000 Pa and 100,000 Pa) and the FFC value is calculated from thedetermined measurement values. The flow behaviour can be characterisedas follows: FFC Flowability >10 free flowing  4-10 slightly flowing 2-4cohesive 1-2 very cohesive  <1 non-flowing

EXAMPLES

The invention is described in more detail hereinafter with the aid ofexamples. These explanations are given simply by way of example and donot restrict the general scope of the invention. Unless otherwisespecified, all polymer powders have a moisture content of less than 5wt. %, and the treatments with the salt solutions were carried out atroom temperature.

EXAMPLES 1-5 Comparison Examples 1-4

Powder of a superabsorbing, partially neutralised acrylic acid polymerpost-crosslinked on the surface was taken from an industrial productionbatch and, without prior screening of the fine fractions (3 wt. % below150 μm), aqueous solutions of aluminium sulfate, iron(III) chloride andmagnesium chloride were added thereto while stirring with a Krupsdomestic mixer, and the whole was slowly thoroughly mixed on a bank ofrollers to break down agglomerates. The quantitative ratio of salt towater as well as the properties of the powders treated with the saltsolutions and also the properties of the original powder withoutscreening (V1) and after screening (V2) of the fine fractions under 150μm are given in Table 1. TABLE 1 Amounts of salt and water in wt. %added to polymer powder Al₂(SO₄)₃*/ Example H₂O FeCl₃ ⁺/H₂O MgCl₂^(#)/H₂O AAP TB SFC V1 23.2 24 52 V2 23.5 24 69 B1 0.1/0.5 22.5 24 53 B20.1/3.0 21.6 23 60 B3 1.0/1.0 21.1 24 105 B4 0.25/0.5 21.6 23 98 B50.75/3.0 20.6 23 120 V3 0.25/3.0 22.1 23 48 V4 0.25/1.0 22.3 23 37*as hydrate with 18 H₂O,⁺as hydrate with 6 H₂O,^(#)as hydrate with 6 H₂O

Comparison Example 5

The same procedure as in V3 is employed except that CaCl₂ is usedinstead of MgCl₂. The polymer powder had a teabag retention of TB=23g/g, an absorption under pressure of AAP=21.9 g/g and a gel permeabilityof SFC=42 [cm³·s·g⁻¹].

EXAMPLE 6

A surface-crosslinked polymer powder according to Example 8 treated withan aluminium sulfate solution is stored under the conditions of theanticaking test for 3 hours at 83° C. and 80% relative atmospherichumidity and then subjected to the screening test. In contrast to anuntreated sample, the polymer powder according to the invention passesalmost quantitatively through the test screen whereas the untreatedsample remains for the most part on the test screen on account of itstendency to agglomerate. TABLE 2 Product Moisture Absorption ScreeningsTreatment [%] [%] without Al₂(SO₄)₃ 10.2 33.0 with Al₂(SO₄)₃ 8.1 99.8

EXAMPLE 7

The powder of a FAVOR® SXM 9100¹ type superabsorber post-crosslinked onthe surface, with 3.9 wt. % of fine fractions below 150 μm and a gelpermeability of SFC =19 [cm³·s·g⁻¹] was homogeneously mixed with 1 wt. %of a 50% Al₂(SO₄) ₃×14 H₂O solution in a Ruberg mixer. Following thisthe polymer powder contained only 2.0 wt. % of fine fractions and thegel permeability SFC had increased to 50 [cm³·s·g⁻¹].

If the fine fractions below 150 gm are separated from the startingpowder not treated with Al₂(SO₄)₃ solution, then a powder is obtainedwith an SFC=43 [cm³·s·g⁻¹].

¹: Surface-crosslinked superabsorber powder of pre-crosslinked,partially neutralised polyacrylic acid from Stockhausen GmbH & Co., KG,Krefeld, Germany.

EXAMPLE 8

Corresponding to the procedure of Example 7, various amounts of 50 wt. %Al₂(SO₄)₃×14 H₂O solution are added to FAVOR® SXM 6565²typesuperabsorber powder post-crosslinked on the surface, containing 3.1 wt.% of fine fractions below 150 μm.

In order to evaluate the abrasion stability the permeability wasmeasured after grinding a sample not treated according to the invention,and one treated according to the invention. The grinding test wascarried out with 10 g of product over 6 minutes in a ball mill at 95r.p.m. The permeability is measured after the grinding test withoutseparating any fine fractions. TABLE 3 Wt. % Al₂(SO₄)₃ Solution 0 0.41.0 2.0 4.0 SFC without separation of the fine fractions 44 52 67 89 87below 150 μm SFC with separation of the fine fractions 71 below 150 μmSFC after ball mill treatment 20 79²Surface-crosslinked superabsorber powder of pre-crosslinked, partiallyneutralised polyacrylic acid from Stockhausen GmbH & Co., KG, Krefeld,Germany.

EXAMPLE 9

A superabsorber powder post-crosslinked on the surface and containing 3wt. % of fine fractions (<150 μm) obtained from pre-crosslinkedpolyacrylic acid partially neutralised to an amount of 70 mol % andgrafted onto polyvinyl alcohol (Mowiol 5-88, 1.9 wt. % dry substance)was treated according to the invention with 1 wt. % of a 50% aqueoussolution of Al₂(SO₄)₃×18 H₂O by mixing on a bank of rollers. The producthad a retention of 25.5 g/g, an absorption under pressure of AAP=21 g/gand a gel permeability of SFC=75 [cm³·s·g⁻¹].

Untreated, the superabsorber including 3 wt. % of fine fractions (<150μm) had a teabag retention of TB=25 g/g, an absorption under pressure ofAAP=22 g/g and a gel permeability of SFC=45 [cm³·s·g⁻¹]. After thescreening of the fine fractions the untreated absorber had, underconstant retention, an absorption under pressure of AAP=22.5 g/g and agel permeability of SFC=75 [cm³·s·g⁻¹].

EXAMPLES 10-13 Comparison Examples 6-7

A superabsorber powder according to Example 1 containing 3 wt. % of finefractions of <150 μm is treated as in Example 1. The properties of theabsorber powders with fine fractions (V6), without fine fractions (V7)and post-treated according to the invention are shown in Table 4. TABLE4 Amounts of salt or water in wt. % added to polymer powder ExampleAl₂(SO₄)₃*/H₂O FeCl₃ ⁺/H₂O AAP TB SFC V6 23 24 45 V7 23.5 23.7 69 B100.5/0.5 21.5 24.5 93 B11 1.0/3.0 22 24 72 B12 0.25/0.5  21 24 76 B130.5/3.0 21 23.5 60*as hydrate with 18 H₂O,⁺as hydrate with 6 H₂O,

EXAMPLES 14-19 Comparison Examples 8-10

A superabsorber powder according to Example 1 containing 5, 10 and 15wt. % of fine fractions of <150μm is treated as in Example 1. Theproperties of the absorber powders with fine fractions (V8-V 10) andpost-treated according to the invention are shown in Table 5. TABLE 5Amounts of salt and water in wt. % added to polymer powder Fine FineFractions Fractions Ex- Before After ample Treatment Al₂(SO₄)₃*/H₂O AAPTB SFC Treatment V8 5 22 24 26 B14 5 0.5/0.5 21.3 24.7 52 B15 5 0.5/3.020.2 24.4 45 2.5 V9 10 21.4 24.3 29 B16 10 0.5/0.5 20.8 24.3 53 B17 100.5/3.0 20 23.7 45 3.5 V10 15 21.5 23.2 25 B18 15 0.5/0.5 19.6 23.7 53B19 15 0.5/3.0 19.8 23.8 40 4.5*as hydrate with 18 H₂O

EXAMPLES 20-24 Comparison examples 11-15

Variously concentrated aqueous solutions of aluminium sulfate were addedto powders of a polyacrylic acid polymer partially neutralised to 70 mol% with sodium hydroxide and post-crosslinked on the surface andcontaining a fine dust fraction <150 μm of 1.1 wt. %, in an MTI mixer(blade mixer) and thoroughly mixed at 750 r.p.m. TABLE 6 Amounts of saltand water in wt. % added to polymer powder Example Al₂(SO₄)₃*/H₂O FFCAAP TB SFC B20  0.15/0.15 14 22.5 27 47 V11 without 5.7 23 26.5 44 B210.15/0.5 11 22.8 26 41 V12 without 6.2 23.3 26.7 37 B22 0.15/1.0 10.422.8 26.5 48 V13 without 6.2 23.5 27 37 B23 0.15/2.0 8.6 22.3 26.8 45V14 without 6.2 24 26.3 38 B24 0.15/3.0 8.3 22 26 63 V15 without 6.823.8 26.5 46*as hydrate with 18 H₂O

The products exhibited a good to extremely good flowability, which inthe case of Examples 20-22 was manifested immediately after briefmixing, whereas the products according to Examples 23 and 24 that hadbeen treated with salt solutions of lower concentration required alonger standing and/or mixing time of up to 1 hour.

1-13. (Cancelled)
 14. Use of a solution of at least one salt of at leasttrivalent cation for restoring the gel permeability of pulverulentpolymers absorbing water or aqueous liquids that have been damaged bymechanical action, said polymers being synthesised from polymerised,optionally pre-crosslinked monomers containing partially neutralisedcarboxyl groups, characterised in that the solution of the salt is addedto the pulverulent polymer after the post-crosslinking and the saidpulverulent polymer and the solution are thoroughly mixed.
 15. Useaccording to claim 14, characterised in that at least one aluminiumcation, iron cation and/or manganese cation is used as cation.
 16. Useaccording to claim 14, wherein the fraction of the cation in the polymerpowder is about 0.001 to about 1 wt. %.
 17. Use according to claim 14,characterised wherein the polymer powder before the addition of thesolution has a moisture content of less than about 10 wt. %.
 18. Useaccording to claim 14, wherein the water content of the polymer powderduring the post-treatment increases by about 0.05 to about 10 wt. % ofthe polymer powder.
 19. Use according to claim 14, wherein the grainfraction is less than about 150 μm before the post-treatment is lessthan about 15 wt %.
 20. Use according to claim 14, wherein the carboxylgroups are about 25 to about 85% neutralised.
 21. Use according to claim14, wherein the polymer contains further comonomers and/or graftpolymers.
 22. Use according to claim 14, wherein the temperature of thepolymer powder before the addition of the solution is from about 0° C.to about 100° C.
 23. Use according to claim 14, wherein the thoroughmixing is carried out with a stirrer operating at a rotational speed ofabout 700 to about 1000 r.p.m. 24-26. (Cancelled)
 27. A pulverulentpolymer suitable for absorbing water or aqueous liquids andpost-crosslinked on the surface, the pulverulent polymer is synthesisedfrom polymerised, optionally pre-crosslinked, partially neutralisedmonomers containing carboxyl groups, wherein the pulverulent polymer hasan absorption under pressure (pressure load 50 g/cm²) from about 19.6g/g to about 23.8 g/g and a gel permeability of from about 26[cm³·s·g⁻¹] to about 120 [cm³·s·g⁻¹].
 28. The pulverulent polymeraccording to claim 27, wherein the pulverulent polymer has been reactedwith a solution of at least one salt of an at least trivalent cation.29. The pulverulent polymer according to claim 27, wherein thepulverulent polymer has been reacted with a solution of at least onesalt of an at least trivalent cation after the post-crosslinking.